The present invention relates generally to bistatic passive differential tracking systems, which is not radar, and specifically, to a process for passively tracking moving vehicles over long distances once their initial position has been established without emitting illumination signals based upon bistatic doppler only target echo return signals engendered by illuminators of opportunity.
The determination of the location of a passive target aircraft can be done with RADAR, which uses a dedicated transmitted waveform from one or more transmitting stations. RADAR requires an active transmission of a specific signal. Alternately, the determination of the location of a passive target aircraft can be done by passive coherent location (PCL) methods, where there is no specific transmission of an electromagnetic waveform required. This has the advantage of being covert, jam-proof, survivable, low cost and reliable. The invention herein is based on PCL methods and is not RADAR.
The term xe2x80x9clocationxe2x80x9d, as used herein, means the identification of a previously unknown locus. The term xe2x80x9ctrackingxe2x80x9d, as used herein, is the activity of following the: motion of a target whose location was previously known. The invention herein deals with tracking only, and requires a prior knowledge of the target""s location. The term xe2x80x9cpassive differential trackingxe2x80x9d is the term used to describe the action of the invention herein, which is the measurement of time-sequential displacement of the instantaneous position of a target with respect to a previously known, independently derived target location. In a bistatic passive tracking system, the illuminating transmitters and receiver are located at different locations.
When used in military applications, monostatic or bistatic radar has the disadvantage that the transmitter cab be detected at long range (hundreds of miles) by the electromagnetic pulses it emits. This allows the enemy to detect the presence of a radar system and also to determine its location. To get around this disadvantage, Passive Coherent Locations (PCL) was developed. PCL does not have a transmitter, but rather has a receiver system that utilizes the radiating emitted by separately located illuminators of opportunity in its reception area. In contrast the transmitter of a radar system which is being used by a monostatic or bistatic radar receive station is specific in waveform, frequency and other parameters.
The task of providing a bistatic radar receiver is alleviated, to some degree by the prior art techniques disclosed in the following U.S. Patents;
U.S. Pat. No. 3,487,462 issued to Holmberg;
U.S. Pat. No. 3,812,493 issued to Afendykiw et al;
U.S. Pat. No. 4,246,580 issued to Caputi Jr.;
U.S. Pat. No. 4,325,065 issued to Caputi, Jr.; and
U.S. Pat. No. 4,370,656 issued to Frazier et al.
All of the patents listed above, disclose bistatic synthetic aperture radar systems, and are incorporated herein by reference. Both of the Caputi patents, as well as that of Frazier et al, disclose Airborne bistatic radar systems entailing a first aircraft possessing a long range radar transmitter, and a second aircraft with a receiver. The distance between the two aircraft,is determined when the second aircraft receives direct path signals, which are received directly from the transmitter on the first aircraft.
The task of tracking targets with a passive bistatic radar tracking system is alleviated by the prior art techniques of the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 2,940,076 issued to Bissett on Jun. 7, 1960;
U.S. Pat. No. 2,968,034 issued to Cafarelli on Jan. 10, 1961;
U.S. Pat. No. 2,972,742 issued to Ross on Feb. 21, 1961;
U.S. Pat. No. 3,863,257 issued to Kang on Jan. 28, 1965;
U.S. Pat. No. 4,281,327 issued to Frazier on Jul. 28, 1981;
U.S. Pat. No. 4,350,984 issued to Fisher on Sep. 21, 1982;
U.S. Pat. No. 4,442,4,32 issued to Quigley on Apr. 10, 1984; and
U.S. Pat. No. 4,477,812 issued to Frisbee on Oct. 16, 1984.
The designs of most prior art bistatic radar systems entail a building of one or more cooperative radar transmitters to serve as illuminators for an especially designed radar receiver. The narrowband passive differential tracking system of the present invention utilizes a receiving system which uses non-cooperative transmitters of opportunity as independent illumination sources and tracks the target with a tracking algorithm which updates the target""s state with respect to a independently known initial position using Doppler-shifted signals reflected from a target.
The process of the present invention is an algorithm which can provide differential tracking of targets based upon a known initial position of the target and the bistatic Doppler target return information. The procedure requires the knowledge of the position of two illuminators or transmitters and the known target initial position. When a target passes through a region illuminated by the transmitters, the present invention can continuously update the target""s new location, based on average speed, course and distance traveled computed from the Doppler shifted target reflected signal history, if the invention is initially given the position coordinates of both illuminators, the receiver and the initial target location.
The process begins by the selection of two illuminators of opportunity, which are two independent electromagnetic signal transmitters which serve as transmitters of uncooperative and independent signals. In the embodiment which was tested, the two illuminators of opportunity were the narrowband video carrier of two television stations: channel 15 of Lancaster, Pa., and channel 16 of Salisbury, Md.
Since most illuminators of opportunity are stationary radio frequency radio frequency (RF) signal sources, their latitudes and longitudes are well established and available. Similarly, the latitude and longitude of the PCL receiver is established. Next, the PCL receiver is monitored until a target vehicle reflects signals to the receiver. At this point, this known initial target position and the subject algorithm are used to establish the change in the target""s state (its position and velocity). This involves using the given latitude and longitude of the two illuminators, the receiver and the target. The present invention uses an interactive narrowband passive coherent location (PCL) two-Doppler tracking program, which continuously accepts new Doppler data as an input for updating the target""s state (position and velocity).
The two-Doppler tracking program of the present invention is an algorithm written in BASIC. This algorithm converts all latitude and longitude values to an x-y grid and correlates the Doppler shifted signals from the two illuminators to an expression of distance in wavelengths which, in turn, can be expressed in more convenient units such as nautical miles. The target position is then updated and reconverted into latitude and longitude, while the change in distance of target location over the time interval provides the vehicle ground speed.
It is a principal object of the present invention to provide a narrowband passive differential tracking system which uses independent sources of electromagnetic emission as illuminators.
It is another object of the present invention to provide a target tracking algorithm which continuously updates a target""s state using Doppler-shifted target echo return signals.
These objects together with other objects, features advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein like elements are given like reference numerals throughout.